[I10n] Add Norwegian (nynorsk) (nn-NO) locale

[ghost]

• I have followed (at least) the PR section of the contributing guide.

[mui-bot]

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• docs/data/material/guides/localization/localization.md

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[oliviertassinari]

@ZeeshanTamboli Note that adding this locale is very borderline to the limit we set for a too-high opportunity cost. The concern is that each local has a cost, so has to be worth the effort.

Starting from https://mui.com/material-ui/guides/localization/#supported-locales

We have

https://www.ethnologue.com/insights/ethnologue200/

and

https://w3techs.com/technologies/overview/content_language

I wouldn't be against reverting, but also why not keep it for now, until it becomes an issue? At least, I think it's important when it comes to future language proposals.

---

Actually, what could be awesome is to

• review https://w3techs.com/technologies/overview/content_language, https://en.wikipedia.org/wiki/List_of_languages_by_number_of_native_speakers, top 100 of https://www.ethnologue.com/insights/ethnologue200/
• see which locales we are missing
• add them under a new table in https://deploy-preview-39481--material-ui.netlify.app/material-ui/guides/localization/
• make it clear that we would love to get help on them; and that won't accept other locales.

Idea moved to #39525.

[ZeeshanTamboli]

Starting from https://mui.com/material-ui/guides/localization/#supported-locales

I noticed that statement in our documentation before giving my approval, but I decided to accept this pull request anyway. As you mentioned, I thought, "Why not keep it?" I'll remember this for future language proposals.

[oliviertassinari]

@ZeeshanTamboli A proposed plan to move forward #39525 (comment). Assuming we can accept x2 more locales than we have today, this PR should be fine.